Natural gas resources vary significantly in compositions. Natural gas contains mostly CH4, but usually also contains various amounts of impurities (e.g. CO2, H2S, N2, heavier hydrocarbons, and water). The concentration of these impurities in natural gas must be reduced to certain level before transportation and distribution. CO2 is the primary impurity in most natural gas resources, and may exceed 50% in some wells under high pressure. Polymers are state-of-the-art membrane materials for CO2 removal from natural gas. Un-crosslinked polymers may be swelled and dilated under high concentration of strongly sorbing penetrant (e.g. CO2 and heavier hydrocarbons). This phenomenon is called plasticization and usually leads to increased membrane permeability and substantially reduced membrane permselectivity.
As a result, un-crosslinked polymers are usually incapable of processing high pressure natural gas with high level of CO2. The problem of membrane plasticization is not limited to the treatment of natural gas. Rather, membrane plasticization may occur in any gas separation application when the gas stream being treated contains a high partial pressure of the selectively sorbed gas component or components. Therefore, plasticization may prevent effective commercial separation of many gases. Plasticization, i.e. swelling of the membrane and the resultant loss in permselectivity, has also been observed in certain carbon molecular sieve (CMS) membranes, as evidenced by, for example, Octavio Salinas et al., High-performance carbon molecular sieve membranes for ethylene/ethane separation derived from intrinsically microporous polyimide, Journal of Membrane Science, vol. 500, pages 115-123 (2006).
In the absence of plasticization, a substantial loss of both membrane permeability and permselectivity would be expected to occur during aggressive gas separation applications due to potential saturation of the micropores. Where the partial pressure of the selectively sorbed gas component(s) is high, it would be expected that the micropores in a CMS membrane may become saturated with the sorbed gases in a relatively short amount of time. Under high CO2 partial pressures, for example, it would be expected that large amounts of CO2 would be relatively quickly absorbed/adsorbed into the micropores of the CMS membrane, which would decrease the pore sizes and reduce the permeability/permeance of the CMS membrane. Thus, one would expect to observe a relatively rapid reduction in both permeability/permeance and permselectivity during treatment of a gas stream in which the sorbed gas component(s), such as CO2, has a high partial pressure. Severe loss in membrane performance due to saturation has been observed, for example, in Cheryl W. Jones & William J. Koros, Carbon molecular sieve gas separation membranes—II. Regeneration following organic exposure, Carbon, vol. 32, issue 8, pages 1427-1432 (1994).